1. Field of the Invention
The present invention relates to a screw compressor.
2. Description of the Related Art
A screw compressor that accommodates a pair of male and female rotors in a compression chamber (rotor chamber) formed in a casing, and that compresses gas drawn from a suction passage by the rotation of the rotors and discharges the compressed gas from a discharge passage is widely used.
Bearings that support rotor shafts of a pair of male and female screw rotors, and shaft sealing members disposed around the rotor shafts to isolate a compression chamber from the other space are necessarily provided for a screw compressor. The bearings are generally fed with lubrication oil. When non-contacting sealing such as dry gas seal is used for shaft sealing parts, so-called seal gas needs to be fed to the shaft sealing parts in many cases. In addition, fluid (e.g., air) compression may generate so-called drain. Therefore, in many screw compressors, passages for feeding oil and gas to their inside and passages for discharging the oil and gas, the drain that is generated in their inside and the like are formed.
For example, Japanese Patent No. 4365443 discloses a screw compressor configured such that oil stored in an oil tank is fed to plural bearings and mechanical seals via an oil feed line including an oil cooler, a pump and a filter. In this screw compressor, in order to form passages that feed lubrication oil to the bearings that support shafts of the screw rotors, a portion of a casing that supports bearings and an outer wall portion thereof are partially and integrally connected in the radial direction.
As in the screw compressor disclosed in the above patent, it is not structurally unnatural to connect the portion that supports the bearings and the outer wall in the radial direction. However, since screw compressors are designed according to various demands, in some screw compressors, the internal portion of the casing and the outer wall portion thereof are not necessarily connected in the radial direction in terms of strength. In some cases, it is more preferable to form passages used for sucking gas into a compression chamber, or to form, in the outside of the bearing, a space that stores a fluid different in kind or pressure from bearing oil, for example, a space that stores oil for lubricating gears that synchronize male and female screw rotors.
A casing of a screw compressor is generally formed by the casting. Although passages and other parts are formed only for passage of a small amount of fluid, passages and other parts that are formed by the casting may not have sufficient thickness in some cases. In these cases, molten metal may not sufficiently spread and may not become desired shape. As a result, the formed passages may communicate with other interior spaces due to insufficient wall thickness. In addition, since the cooling speed becomes uneven, material defects may be caused, resulting in breakage. Therefore, for conventional screw compressors, an internal portion of a casing and an outer wall portion are integrally formed in the radial direction only for securing fluid passages, even if there is no mechanical requirement. Thus, the casing becomes heavy and large.